It is well known to separate air into oxygen and nitrogen products by cryogenic distillation. It is conventional to liquify some or all of the nitrogen product taken from the distillation means. The liquefier may be independent of or integrated into the air separation plant. A liquid oxygen product may likewise be produced.
The present invention relates to an integrated air separation-nitrogen liquefaction process and plant. Such a process and plant is disclosed in UK Patent Specification 1 258 568. The disclosed process utilizes a single distillation column to separate incoming air into oxygen and nitrogen. Reboil for the bottom of the distillation column is provided by a high pressure nitrogen stream which, after condensation in the reboiler, is sub-cooled and used partly to provide reflux for the distillation column and also to provide liquid nitrogen product. Refrigeration for the plant is provided by removing portions of the high pressure nitrogen upstream of the reboiler and expanding each such portion in a turbine. It has been found that this arrangement is relatively inefficient thermodynamically. A further inefficiency lies in the fact that the process includes a second distillation column to separate a crude argon stream from an argon-enriched oxygen stream withdrawn from the first distillation column. This second distillation column is inefficient partly because there is no reboiler provided at the bottom thereof.
In contrast thereto, the present invention provides a process and apparatus utilizing an improved cycle for effecting reboil of a distillation column employed to separate air into oxygen and nitrogen which provides reflux for the distillation column and refrigeration for the liquefaction of the nitrogen. The present invention further provides improvement in the operation of an argon column associated with a distillation column or columns for separating air into oxYgen and nitrogen.